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The Planets this Month - October 2013

Mercury

Mercury continues to remain well placed for observation during the first three weeks of October for southern hemisphere and tropics based observers. During this time the innermost planet is visible just after sunset south of due west. Peak latitude occurs on October 9th, the date of greatest elongation east (25 degrees). On this date, Mercury from latitudes of 35S (approx. equal to Sydney, Cape Town and Santiago) is 20 degrees above the horizon, 30 minutes after sunset. Indeed, for these observers this is the most favourable evening apparition of the year.

Mercury is at its brightest at the start of the October before fading as the month progresses. From October 1st to 24th - the visibility period - Mercury fades gradually from magnitude -0.1 to +1.0. On October 6th, the extremely thin waxing crescent Moon passes 3 degrees north of Mercury and on October 10th, Mercury (mag. -0.1) passes 5 degrees south of Saturn (mag. +0.6).

Unfortunately, from northern temperate latitudes during October, Mercury is not suitably placed for observation.

Venus

Venus, the brightest of all planets, remains a stunning object in the early evening sky from the southern hemisphere and the tropics. During October the planet brightens from magnitude -4.2 to -4.4 and sets nearly 4 hours after the Sun from these latitudes. No other planet or star (possible exception a supernova) comes close to matching the apparent brilliance of Venus when seen from Earth. During October, the phase of Venus decreases from 63% to 50% (half phase or dichotomy).

Unfortunately, Northern Hemisphere based observers are not so lucky. The planet is visible during October but due to the angle of the ecliptic it remains inconveniently low down above west-southwest horizon, setting just over 1 hour after the Sun.

Venus starts October in Libra before crossing into Scorpius on October 7th and then into Ophiuchus later in the month. On October 3rd, Venus reaches aphelion when it's 0.728 AU (approx. 109 million km or 67.7 million miles) from the Sun.

The waxing crescent Moon passes 5 degrees north of Venus on October 8th. Later in the month (Oct 16th), Venus passes 2 degrees north of the brightest star in Scorpius, red supergiant Antares (α Sco - mag. +1.0). The difference in brightness between the two is striking with Venus more than 100x brighter than Antares.

Mars

Mars is an early morning object in Leo during October. From northern temperate latitudes, it's now visible towards the east for 4 hours before sunrise at the start of the month, improving to over 5 hours by months end. The planet is rather less favourably placed for those living further south.

The distance between Earth and Mars is currently slowly decreasing as our planet on its faster inner orbit closes the gap, before the pair reach their closest separation around April 2014, the time of Mars' next opposition. Consequently during October, Mars improves from magnitude +1.6 to +1.5 and its apparent size increases from 4.4 to 4.9 arc seconds. Telescopically, the apparent size is still small so spotting details remains difficult but the situation is gradually improving.

On October 1st, the waning crescent Moon passes 7 degrees south of Mars and again on October 30th when it passes 6 degrees south of the planet. In between these two dates, a nice conjunction occurs on October 14th when Mars passes 1 degree north of Regulus (α Leo). At magnitude +1.4, Regulus is marginally brighter than Mars (mag. +1.5) but the most striking aspect is the colour difference, deep red-orange Mars against blue-white Regulus.

Jupiter is now a brilliant object amongst the stars of Gemini. The "King of the planets" rises just after midnight at the start of the month from northern temperate latitudes, improving to before midnight by months end. For locations much further south, Jupiter rises a couple of hours later.

During October, Jupiter brightens from magnitude -2.2 to -2.4 and it's apparent size increases from 38 to 41 arc seconds.

On October 25th, the waning gibbous Moon passes 5 degrees south of Jupiter.

The early part of October offers a last chance to glimpse Saturn before it reaches solar conjunction in November. From northern temperate latitudes "the Ringed planet" may be glimpsed low down towards the west-southwest after sunset, but it won't be long before the planet is lost to the bright twilight. Those based in the tropics and southern hemisphere have it better and should be able to spot Saturn for a while longer before they also lose sight of it before months end.

During October, Saturn is located in Libra and shines at magnitude +0.7. On October 7th, the waxing crescent Moon passes 2 degrees south of Saturn and on October 10th, Mercury (mag. -0.1) passes 5 degrees south of Saturn. Both events occur in the early evening sky.

Uranus

Uranus reaches opposition on October 3rd in Pisces and hence is visible all night during October. At magnitude +5.7, the planet is faintly visible to the naked eye under dark skies and good conditions. Even if you can't see Uranus with the naked eye it's a very easy binocular target.

At opposition, Uranus is located approx. 19.040 AU or 2848.3 million km (1769.9 million miles) from Earth. The planet has an apparent diameter of 3.7 arc seconds and a small telescope at high magnification will show the planet as a small green disk, obviously non-stellar. However, even when viewed through the largest amateur telescopes it's difficult to make out any details on the surface of Uranus.

Uranus passes only 6 arc minutes north of mag. +6.4 orange star 93 G. Psc (HD 3457) on October 15th. Through a telescope at medium / high magnifications the pair will appear like a close double star with Uranus the slightly brighter of the two.

Neptune is located in Aquarius and is visible as soon as it's dark enough and remains so for the majority of the evening. It's positioned almost 3 degrees west of Sigma (s) Aquarii (mag. +4.8). The solar systems outermost planet fades slightly during October from magnitude +7.8 to +7.9. Although not visible with the naked eye it can be seen with binoculars and small telescopes.

The waxing gibbous Moon passes 6 degrees north of Neptune on October 15th.